Yiyuji 疑獄集 "Collection of controversial lawsuits" is a collection of law cases and verdicts compiled by He Ning 和凝 (898-955) and his son He Meng 和㠓. The book was compiled between 923 and 929, when He Hing was Vice Director in the Ministry of Justice (xingbu yuanwailang 刑部員外郎) under the Five Dynasties 五代 (907-960). The text was enlarged by two fascicles and published by his son around 990. The original book with a length of 3 juan and 67 paragraphs is lost, but it could be reconstructed by Li Songxiang 李嵩祥, who published a 10-juan-long edition in 1535. In 1851, Jin Fengqing 金鳳清 reproduced the original text after revision. He also added 30 further cases in an appendix. The title of the book goes back to a judicial term dating from the late Warring States period 戰國 (5th cent.-221 BCE).
In his preface to a commentary on Song Ci's 宋慈 (1182-1246) collection Xiyuanji 洗冤集, Mei Qizhao 梅啟照 (1826-1894) says that in antiquity there were specialist texts for the inspection of cases called yiyuji. They recorded very difficult cases that required a detailed research and could not easily be judged. Only when a fair judgment was considered as helpful for the future, such cases were rated as precedents. There are a hundred of such cases known that each bear four-syllable titles that give these cases an individual character like a story, like Li Chong huan er 李崇還兒, Ji Bing bian ying 丙吉辨影, Huang Ba lu luan 黃霸戮亂, Yan Zun bi ting 嚴遵壁聽 or Zhao He ji chan 趙和籍產.
These cases can be divided into three groups, first such that describe careful methods of objective investigation (zhi yu zhi dao 治獄之道 "the way of managing lawsuits"), second, such that stress a personal and fair investigation by the judge who had to rely on evidence (shen an zhi dao 審案之道 "the way of examining cases"), and third, precedents describing concrete methods of investigation (jian yan zhi fa 檢驗之法 "methods of investigation"), partially relying on forensic expertise, for instance, whether a victim was killed with a bamboo stick, or whether a crime could have committed by a left-handed person; whether an injury was real or self-inflicted, and with which kind of object.
The Yiyuji is an important collection that proves the state of jurisdiction in ancient China.
A supplement called Bu yiyu ji 補疑獄集 was compiled by Zhang Jing 張景 during the Ming period 明 (1368-1644). The 6-juan-long text includes 182 more cases of unjust verdicts, in order to unveil corrupt practices among the local officialdom. In most editions it is appended to the Yiyuji.